1. Field of the Invention
The invention relates to a monitor for monitoring the position of the path of motion of a reciprocating piston relative to the surrounding cylinder, including a sensor which communicates with an evaluation unit and which is fixedly arranged in the region of the dead center of the piston movement relative to the cylinder, and an indicator unit arranged in the region of the face of the piston, whereby the indicator unit is disposed in the region of the dead center of the piston movement in a lateral space that can be evaluated by the evaluation unit to determine the relative position of the path of motion of the piston and which lateral space can be detected by the sensor.
2. The Prior Art
Arrangements of the aforementioned type are used in reciprocating piston machines, particularly reciprocating piston compressors, to absolutely prevent metal-to-metal contact of piston and cylinder wall, which can lead to premature operational failure and expensive damages. Positioning and guiding of the reciprocating piston by means of separate rider rings or its own piston guide, as basically provided for this object, are subject to corresponding wear, especially in dry-running piston compressors, which determines the operational life of the compressor. Corresponding monitoring is required even with lubricated piston compressors to prevent costly damages on pistons and cylinder liners with lengthy non-operational periods because of failure in lubrication and subsequent abrasive wear as a result of dry friction. A prognosis of wear of the guides or the rider rings or the like is necessary for condition-based maintenance so that the available remaining operational life can be estimated with a degree of certainty. A shut-off element is often provided to protect the machine when a minimum distance is exceeded between the piston and the inner cylinder wall.
Direct measurements of the distance between the piston and the inner cylinder wall, as disclosed in U.S. Pat. Nos. 6,318,147 B1 and 4,143,319 A, are practically never used because of the exposure of the measuring point (pressure, temperature, deposits, corrosion, risk of explosion and the like) and the thereby related disadvantages. In U.S. Pat. No. 4,987,774 A is disclosed, for example, measuring or monitoring of the wear of rider rings of horizontal reciprocating compressors through determination of the descent in position of the piston rod by means of eddy current sensors. Measuring is performed by means of a sensor arranged outside of the cylinder at a location that is accessible relative easily whereby a relationship to the relevant distance of the piston on the piston rod and the inner cylinder wall is performed through triangulation of the measured value. This method is relatively inaccurate and provides only relatively unreliable information which cannot be used for the protection of the machine based on the multiplication of errors through the required triangulation as well as the negative influences of the movement of the cross head, bending of the piston rod caused by the movement biasing the piston, vibration of the piston rod, different magnetic and electric properties of the piston rod surface as a result of contaminating deposits and the like. For accurate measurement there would be additionally necessary complicated temperature compensation of the measurement signals of the eddy current sensor monitoring the distance of the piston rod sliding alongside, which would create further difficulties in practice.
JP-62-151703 A additionally discloses a method or device of the aforementioned type with an accuracy advantage in principle since possible contact between the piston and the inner piston wall of reciprocating piston machines occur, in general, interestingly at the outer, cover-side end of the piston at first whereby the determination of radial shifting of the piston or a component fixedly connected thereto (such as the piston rod protruding through the piston) has basic advantages in the region of the respective dead center of the path of motion in a plane essentially orthogonal relative to the axis of the piston. In the arrangement disclosed in the above-mentioned patent, there is an optical sensor provided in the cylinder head which evaluates the reflection of a beam of light directed onto an indicator unit on the front end of the piston rod whereby the beam of light is reflected in an altered manner corresponding to the concentrically arranged depressions and elevations of the indicator unit and the shifting thereof. One disadvantage of this known arrangement can be essentially seen in the fact that the optical measurement of the light reflection or the thereby resulting measurement signal is extremely susceptible to contamination by deposits on the sensor and on the indicator unit so that obtained measurement results give again unreliable information about the actual position of the path of motion of the piston relative to the cylinder. Moreover, the sensor is also directly exposed to the unfavorable ambient conditions existing at the measuring point (pressure, temperature, corrosion and the like) which greatly reduces also the operational life and greatly limits the range of employment of such arrangements in addition to the problems associated with the seal of the pickup bore for the sensor in the cylinder head extending all the way to the outside.
It is the object of the present invention to improve on an arrangement of the aforementioned type in such a manner that the described disadvantages of the known arrangement are avoided and that the position of the path of motion of the reciprocating piston relative to the cylinder or its spatial shifting can be detected in a simple and robust manner to make possible a dependable determination of the distance from the piston to the surrounding cylinder wall.